In the 1990s, the practical application of a MR (Magneto-Resistive effect) head and a GMR (Giant Magneto-Resistive effect) head has contributed to the skyrocketing of the recording density and capacity of an HDD (Hard Disk Drive). However, since the problems of heat fluctuation of a magnetic recording medium became conspicuous in the early 2000s, the speed of the increase in recording density has slowed down temporarily. Even so, a perpendicular magnetic recording was put in practical use in 2005, the perpendicular magnetic recording being more advantageous to high density recording theoretically than a longitudinal magnetic recording. This event triggers a recent growth rate of 40% for the recording density of HDD.
According to a latest demonstration experiment for HDD, 400 Gbits/inch2 has been attained. If this trend continues strongly, a recording density of 1 Tbits/inch2 is expected to be attained around 2012. However, it will not be easy to attain such a high recording density even employing the perpendicular magnetic recording, because the problems of heat fluctuation will still become conspicuous.
A “high-frequency magnetic field assist recording method” is proposed as a recording method which can solve this problem (U.S. Pat. No. 6,011,664). In the high-frequency magnetic field assist recording method, the magnetic field with a frequency sufficiently higher than a recording signal frequency near the resonant frequency of a magnetic recording medium is locally applied to the medium. As a result, the medium resonates, and a portion of the medium, to which the high frequency magnetic field is applied, has a coercivity half or less than that of the medium, to which no filed is applied. According to this effect, it is possible to write into a magnetic recording medium with a higher coercivity and higher anisotropy energy (Ku) by superimposing the high frequency magnetic field onto the recording field thereof. However, the method disclosed in U.S. Pat. No. 6,011,664 employs a coil to generate the high frequency magnetic field, making it difficult to efficiently apply the high frequency magnetic field to the medium.
Consequently, a method employing a spin torque oscillator has been proposed (see, for example, US-A20050023938, US-A20050219771, US-A20080019040, IEEE Trans. On Magn., Vol. 42, No. 10, PP. 2670). In the method disclosed, the spin torque oscillator includes a spin injection layer, a intermediate layer, a magnetic layer and electrodes. A direct current is passed through the spin torque oscillator via the electrodes to cause ferromagnetic resonance of magnetization in the magnetic layer, the ferromagnetic resonance being due to spin torque by spin injection. As a result, the spin torque oscillator generates the high frequency magnetic field.
Since the spin torque oscillator is about tens of nm in size, the high frequency magnetic field generated localizes in an area of about tens of nm near the spin torque oscillator. Furthermore, the in-plane component of the high frequency magnetic field allows it to cause the ferromagnetic resonance in a perpendicularly magnetized medium and to substantially reduce the coercivity of the medium. As a result, a high-density magnetic recording is performed only in a superimposed area of a recording field generated from a main magnetic pole and the high frequency magnetic field generated from the spin torque oscillator. This allows it to use a medium with a high coercivity (Hc) and high anisotropy energy (Ku). For this reason, the problem of heat fluctuation can be avoided at the time of high density recording.
In order to make a recording head for the high-frequency magnetic field assist recording, it becomes important to design and produce the spin torque oscillator capable of providing a stable oscillation with a low driving current and generating an in-plane high-frequency magnetic field to sufficiently cause a magnetic resonance of the magnetization in the medium. However, in order to acquire a very strong high frequency magnetic field, a large current must be applied to a spin torque oscillator. The large current gives rise to heating of the spin torque oscillator to deteriorate the performance thereof as a result of the heating. For this reason, a novel method is required to realize the high-frequency magnetic field assist recording using a weaker high-frequency magnetic field, i.e., a lower intensity high-frequency magnetic field.